Shadow-mask-based deposition is a process by which a material is deposited onto the surface of a substrate such that the deposited material is patterned as desired during the deposition process itself. This is often referred to as “directly patterned” the patterned layer of material.
In a typical shadow-mask deposition process, the desired material is vaporized at a source that is located some distance from the substrate. As the vaporized atoms of the material travel toward the substrate, they must pass through a shadow mask that is positioned just in front of the substrate surface. The shadow mask contains openings (i.e., apertures) whose arrangement matches that of the desired pattern for the material on the substrate (in similar fashion to a silk screen or art stencil). As a result, the vaporized atoms pass only through the apertures to deposit on the substrate surface.
Shadow-mask-based deposition has been used in the integrated-circuit (IC) industry to deposit patterns of material on substrates for many years, due, in part, to the fact that it avoids the need for patterning a material layer after it has been deposited. As a result, its use eliminates the need to expose the deposited material to harsh chemicals, such as acid-based etchants, caustic photolithography development chemicals, and the like, to pattern it. In addition, its use also reduces the amount of handling and additional processing to which the substrate must be subjected, thereby potentially reducing substrate breakage and increasing fabrication yield. For many materials, such as organic materials, patterning by shadow mask is virtually a necessity because the materials cannot be subjected to photolithographic chemicals.
Unfortunately, the feature resolution that can be obtained by shadow-mask deposition is diminished due to the fact that the deposited material tends to spread laterally after passing through the shadow mask—referred to as “feathering.” As a result, critical features must be separated by relatively large areas of open space between them. In many applications, this has limited the density of overall device resolution that can be obtained.
For example, active-matrix organic light-emitting-diode (AMOLED) displays require shadow-mask-based deposition of their light-emitting material because these materials cannot be subjected to photolithography or etching. For full-color AMOLED displays, each display pixel includes several regions—referred to as “sub-pixels” of light-emitting material, each emitting a different color. Due to feathering issues, however, relatively large safety-margin gaps must be included between these subpixel regions to ensure no overlap in deposited materials. In some cases, these gaps must be nearly as large as the subpixel itself which introduces undesired optical artifacts—particularly when viewed in near-to-eye applications, such as head mounted displays. Prior-art AMOLED displays, therefore, have typically been restricted to approximately 600 pixels-per-inch (ppi) or less which is insufficient for many applications, including near-to-eye augmented reality and virtual reality applications. In addition, the need for large gaps between subpixels gives rise to reduced pixel fill factor, which reduces display brightness. As a result, the current density through the organic layers must be increased to provide the desired brightness, which can decrease display lifetime.
An alternative approach is to use a shadow-mask with an aperture as large as the active area of the display itself to deposit a monochrome white-emitting organic layer across the entire display and then to pattern or deposit red, green and blue color filters on top of the OLED. These color filters absorb all of the emitted white light except for the red, green or blue portion of the spectrum (depending on the color filter), allowing a full color image to be created. However, these color filters absorb up to 80% of the emitted light which significantly reduces display brightness, again requiring operation at higher than desirable drive currents.
The need for a process that is suitable for directly patterning high-resolution patterns of material on a substrate remains unmet in the prior art.